Thick film conductors serve as electrical interconnections between resistors, capacitors, and integrated circuits. Thick film conductors are used in the microcircuit industry to terminate resistor or capacitor patterns, typically by firing the conductor pattern on a substrate and then printing the resistor or capacitor pattern over part of the conductor pattern and then firing them both.
The use of gold for thick film circuit fabrication has resulted in a significant advance in microelectronics. Gold is the most malleable and ductile of all metals, properties that assist in drawing fine wire and in wire bonding. Furthermore, gold does not readily combine with oxygen. The absence of an oxygen reaction allows air firing of gold at temperatures up to its melting point. However, a well known problem with the use of gold for printing inks for thick film conductors is that it is difficult to obtain adhesion between the gold and underlying substrate. In particular, while the lack of an oxide layer on the surface of gold is advantageous when forming a bond with a metal, such as in wire bonding, it is believed to be disadvantageous for adhesive bonds with metal oxides such as glasses or ceramics.
In the present invention it is shown that the addition of copper oxide which easily reduces, results in metallic copper during the firing cycle. The copper alloys with the gold and heightens its oxygen affinity. Thus, a stronger bond is achieved with an oxidic substrate. This theory will also hold true with the substitution of copper oxide and with other easily reduceable metal oxides.
Further, most existing gold thick film compositions contain cadmium compounds as essential ingredients. Empirical studies have established that cadmium compounds such as CdO and glasses containing CdO are essential for the strong adhesion of thick film gold compositions to ceramic bodies. Recently, the use of cadmium containing compositions has been restricted, and in some regions it will be banned in the near future. More restrictions on the use of cadmium containing compounds are expected because of their high health hazards.
Nontoxic lead-free and cadmium-free frit systems which are low melting, moderate in expansion and durability that provide excellent wetting are not, however, known in the art. Therefore, consistent with efforts to reduce or eliminate lead and cadmium from broad catagories of products containing glass frits, the present invention deals with a lead-free and cadmium-free glass frit that has been shown to be useful in the formulation of thick film paste compositions. The composition of the present invention renders a nontoxic, cadmium-free/lead-free thick film alternative to presently used compositions.